Position determination devices, such as hand-held GPS (global position system) receivers are widely used and well known in the art. Such devices typically receive signals from a plurality of GPS satellites, perform complex measurements on the received signals, and analyze various other information received from the satellites (e.g. ephemeris data). These steps are performed by conventional position determination devices in order to compute the location of the position determination device. It will be understood that in order to perform the aforementioned complex measurements and signal analyses, a conventional position determination device must have sophisticated hardware and system components. The sophisticated hardware and system components are a source of significant cost in such position determination devices.
In addition to having each position determination device operate independently, some prior art approaches employ a networked system. In a conventional networked system, typically each position determination device (sometimes referred to as a rover unit) is coupled to a central base. In many instances, the user of the rover units may subscribe to or pay a fee to access the networked system. Upon computing their respective positions, the numerous position determination devices or rover units report their position to the central base. As a result, the central base (e.g. a dispatch station) is aware of the respective locations of the various rover units. The position information of the rover units is often made available to the other rover units via the central base. In such a networked system, the number of rover units can vary widely. For example, a large percentage or a small percentage of the eligible or subscribing rover units may be accessing the system at a given time. Additionally, the networked system may periodically be receiving new subscribers. That is, a new user may purchase a new rover unit and then attempt to utilize the networked system.
One prior art example of networked system is described in U.S. Pat. No. 5,663,734 to Krasner, entitled "GPS Receiver and Method for Processing GPS Signals." The Krasner reference illustrates several of the disadvantages associated with prior art networked systems. For example, the base station commands or directs the remote unit to perform various functions, such as reporting the remote unit's location to the base station. As a result, a considerable processing and control burden is placed on the base station. Additionally, in the Krasner system, the maximal range that the remote unit can extend from the base station is limited to approximately 1/2 the speed of light times the PRN (pseudo-random noise) repetition period (1 millisecond). This distance is calculated to be approximately 150 kilometers. The relatively short maximal range limits the usefulness of conventional networked systems such as the system of the Krasner reference. That is, operation of the rover units is restricted to only a limited distance from the base station.
Another drawback, during normal use (e.g. when first turned-on) a conventional position determination device/rover unit undergoes a time-consuming initialization process. During this process, the rover unit gathers substantial initialization data from the GPS satellites. The initialization data is required for the position determination device to be operable and in condition to determine position information. Additionally, the initialization process is often confusing to the average consumer. The drawbacks associated with a conventional time-consuming initialization process are further compounded in a networked system due to the constant addition of new subscribing rover units to the network. That is, each of multiple new subscribing rover units has to be completely initialized before use, and multiple new users are often confused by the initialization process.
In addition to the aforementioned drawbacks, a rover unit must be very inexpensive in order for the rover unit to be attractive to and affordable for the average consumer. Therefore, it is not commercially feasible to attempt to resolve any of the aforementioned drawbacks by significantly increasing the complexity, and, correspondingly, the cost of the system and hardware components of the rover unit. Previous self-contained GPS receivers could do everything necessary to obtain a position fix, but at significant expense. It would be desirable to reduce the costs of obtaining position in a device that already includes a communications system.
Thus, a need has arisen for a method and system for improving the operation of a rover unit in a networked system. Moreover, a need exists for a method and system for improving the operation of a rover unit in a networked system without significantly increasing the system and hardware requirements of the rover unit.